the fundamentals of iv (current/voltage) measurement

The Fundamentals of IV (Current/Voltage)

Measurement

Alan Wadsworth Market Development Manager Americas Field Marketing

April 10, 2012

Webcast Agenda

• IV Measurement Basics • Source/Measure Units (SMU) Introduction • Low Current Measurement Techniques • Making Accurate Resistance Measurements • B2900A SMU Series Features & Benefits • Final Remarks

IV MEASUREMENT BASICS

What is Current?

Negative charge (electrons) flowing in the negative x direction is equivalent to positive charge flowing in the positive x direction.

Note: In semiconductors it is possible to measure the flow of positively charged electron holes.

- - + -

x

- - -

- - -

- -

Understanding Accuracy & Repeatability

High accuracy, low repeatability Low accuracy, high repeatability

Accuracy – The degree of conformity of a measured or calculated quantity to its actual (true) value. Repeatability (aka precision) – The degree to which repeated measurements or calculations show the same or similar results.

Understanding Measurement Resolution

Simplified analog-to-digital converter (ADC) circuit.

Resolution – The smallest change in data that an instrument can display.

The number of bits available to the digital-to-analog converter (DAC) determines the fineness of the measurement detail.

Example: 20 bits of resolution represents the ability to distinguish one part in 220 or 1,048,576.

What is a Half Digit of Resolution?

Be careful! This can mean different things for different instruments

Question: What is the ½ digit here?

Answer: It is the 7. In this case the least significant digit only has ½ the resolution of the other digits.

Why Are Triaxial Cables Needed for Low-Current?

BNC (Coaxial) Cable: Triaxial Cable:

Leakage Current: Leakage Current:

Triaxial cable reduces leakage current by a factor of 100,000,000.

What is a 4-Wire (Kelvin) Measurement?

Rcable

Rcable

Rcable

Rcable

IForce

+ - VSense

I=0 I=0

Force Line 1 Force Line 2

Sense Line 1 Sense Line 2

RDUT

Eliminate cable resistance from the measurement

Example Instrument Outputs (Banana Jack)

Sense Force Guard High

Low

± 250V = Max

± 210V = Max

By default the low outputs are tied to chassis ground, but in this case they can be floated up to 250 V above or below chassis ground if desired.

This tells you the maximum allowable voltage (210 V in this case) between the high and low inputs.

When making a basic 2-wire measurement you should use the Force outputs.

4-wire measurements require the use of the Sense lines in addition to the Force lines.

SOURCE/MEASURE UNIT (SMU) INTRODUCTION

What is a Source/Measure Unit (SMU)?

Note: The tight integration of these measurement resources yields better accuracy and faster measurement than would an equivalent collection of separate instruments.

Simplified equivalent circuit (2-wire measurements):

Ideal Voltage Source

Ideal Current Source

Ammeter

Voltmeter Chassis Ground

A

V

+ -

+

-

High Force

Low Force

+ -

Why Would You Use an SMU for IV Measurements?

VFIM (Force voltage & measure current)

IFVM (Force current & measure voltage)

ADUT DUTV

An SMU integrates the following capabilities into each channel: • Four-quadrant voltage source • Four-quadrant current source • Voltage meter • Current meter

Here are the two most common modes of operation:

Why Use a Benchtop SMU for IV Measurements? Reason #1: Improved Measurement Efficiency

Multimeter

Multimeter

DC Voltage Source

DC Voltage Source

DUT

RL

Bench-top setup example for 4-terminal DUT

IN OUT DUTIN OUT

B2900A setup example for 4-terminal DUT

Channel 2 on the back side

Channel 1

Problem: Limited bench-top space for single-function instruments. Solution: A benchtop SMU reduces the number of instruments and reduces

messy wiring.

Non-SMU setup example for 4-terminal device

SMU setup example for 4-terminal device

Why Use a Benchtop SMU for IV Measurements? Reason #2: Eliminate the Need for a PC

Problem: Many bench-top instruments require a PC to graphically display data and to make measurements.

Solution: A GUI-based benchtop SMU supports real-time IV curve monitoring directly on the instrument. You can also save graphs and data to a flash drive for report generation.

Dump toUSB memoryMeasurementSet SMUs for DUT

Quick Evaluation

Report

Organize your quick report

LOW CURRENT MEASUREMENT TECHNIQUES

Low-Current Measurement Challenges

???

• How do I eliminate electromagnetic interference?

• How do I avoid creating ground loops?

• What is measurement ranging and how do I optimize it?

• Why is integration time important in eliminating noise?

• How do I eliminate voltage and current transients?

Use Shielding to Avoid Electromagnetic Interference

Shielding Box

Probing Equipment and DUT

Note: Shielding is not the same as guarding.

Avoiding Ground Loops

Conductive planes tied together at only one point cannot have any current flowing between them.

Conductive planes tied together at multiple points creates a loop for current (a condition to be avoided).

Do not connect up equipment to ground at more than one point!

Understanding Measurement Ranging - 1

All measurement circuitry needs to switch in and out various resistors in order to measure currents and voltages at different levels. The measurement range determines the maximum measurement resolution that you can obtain*.

*Typically 5 decades below the measurement range.

Understanding Measurement Ranging - 2

10 pA

100 pA

1 nA

10 mA

100 mA

Fixed Ranging – Always stay in the same measurement range

Limited Ranging – Never go below the specified range limit

Auto Ranging – Go as low as necessary to make an accurate measurement (down to the lowest range supported if necessary)

Current Measurement Range

Integration Time Eliminates Measurement Noise AC Voltage

Time

Time

Measured Value Averaged over multiple power line cycles

Integration DOES NOT have any effect on the measurement resolution.

Eliminating Transient Effects

Delay Time

SMU Output

Time

Hold Time

Measurement

The hold time and delay time settings allow you to specify how long to wait before starting a measurement after the SMU applies voltage or current.

Step #1: Perform Self-calibration

Before performing Self Calibration After performing Self Calibration

1 nA

1 fA

1 pA 100 fA

Almost all instruments designed for low-current measurement have some sort of self-calibration mechanism. It is important that you DO THIS before attempting a low-current measurement. Note: B2900A example shown

Press the System > Cal/Test function keys

From the front panel:

Press OK to perform Self-calibration

Step #2: Select the Correct Current Measurement Range

Using 1 µA Current Measurement Range: Using 10 nA Current Measurement Range:

1 nA

1 fA

10 pA 100 fA

From the front panel: In Single View mode you can specify the current measurement range.

Most instruments DO NOT boot up in their lowest measurement range. In this example notice the improvement in measurement performance obtained by changing from the 1 µA current measurement range to the 10 nA current measurement range. Note: B2900A example shown

Step #3: Increase the Integration Time to Eliminate Measurement Noise

Using SHORT(0.01 PLC) integration time: Using NORMAL (1 PLC) integration time:

1 nA

1 fA

1 pA 100 fA

From the front panel: In Single View mode you can select the measure-ment speed (integration time)

In general, low-current measurements need at least 1 power line cycle (PLC) of integration to obtain decent results (in this example NORMAL integration). Extremely low currents and/or noisy environments may require LONG integration (16 PLCs). You can use MANUAL integration to select PLC values between these two extremes. Note: B2900A example shown

Step #4: Select an Appropriate Measurement Trigger Delay Time

Using a Trigger Delay Time of 0 ms Using a Trigger Delay Time of 200 ms

1 nA

1 fA

5 pA 100 fA

From the front panel: In Single View mode you can select the measure-ment delay time

The length of the wait time depends primarily on the size of the voltage step; larger voltage steps require longer wait times. However, the magnitude of the capacitance being driven also impacts the wait time (larger C longer wait times). Note: B2900A example shown

Important! Current Measurements <1 nA Require Triaxial Cabling & Fixturing

Kelvin Adapter Non-Kelvin Adapter

N1294A-001 Banana–Triaxial Adapter for 2-wire connection (non-Kelvin) N1294A-002 Banana–Triaxial Adapter for 4-wire connection (Kelvin)

MAKING ACCURATE RESISTANCE MEASUREMENTS

L

W

d LWdR

LAR ==ρ

+

-

IVR = V

I

Understanding Resistivity and Resistance

When characterizing materials we typically want to know the resistivity, which is a material property that is independent of the dimensions of the device being measured.

=

)||()||( SSSSS RRRRR +=

Rs Rs

Understanding “Ohms per Square”

You may sometimes see the term “Ohms per Square”. This is simply another way of specifying resistivity. As long as the thickness of a material is constant, it has the same measured resistance for any sized square of the material.

Measuring Resistance: A Trivial Measurement?

V = I x R, correct? NO!

V = I x R(T) Resistance depends on temperature!

Resistance measurement is a tricky balance between two factors:

1. To keep the resistor from heating up and the resistance value from changing, we need to keep the current (= power) low.

2. Small currents imply that we need to measure smaller voltages, which in-turn requires more voltage measurement resolution capability.

This is referred to as the Joule self-heating effect.

SiO2

1 cm 0.014 W/oC-cm2

(h × 71) oC-cm2/W, or 0.007 oC-cm2/W for h = 1 µm

SiO2

1 cm

h

The Importance of Thermal Resistance

• For a 1 cm2 block of SiO2 1 µm thick, if you apply 1 W the temperature will rise by 0.007 oC

• For a 10 µm2 block of SiO2 1 µm thick, if you apply 1 W the temperature will rise by 7,000 oC!

How Much Power Can I Apply to a Structure?

RV

RVVIVP

2

=×=×= )(maxmax TRPV ×=

At or near room temperature the resistance of a Cu or Al metal line changes by about 0.35%/oC. This allows us to compute the maximum amount of power that can be dissipated to produce a 0.1% change in resistance for a Cu or Al metal square 10 mm by 10 mm.

mWPcmmmmmCWcmCP oo

04.0)1/10()10/(1/%35.0/007.0%1.0

max

222max

=∴×××−×=

To achieve 0.1% accuracy in a copper structure with an equivalent resistance of 10 mΩ per square (1 mm thick film) we have the following:

VmmWV 000632.0)10()04.0(max ≈Ω×=

Need 1 µV of voltage measurement resolution!

Thermo Electro-Motive Force (EMF): What is It?

EMF: A transient voltage pulse that is associated with reed relay switches.

Thermo-EMF example of general reed relay

-1.E-05

0.E+00

1.E-05

2.E-05

3.E-05

0 500 1000 1500 2000Time (s)

Vm

(V)

Note: This is NOT an example of the relays used in our instrumentation.

EMF can have a significant impact on low-level resistance measurements.

Modified Kelvin Resistance Measurement Key Points: 1) Make sure that you eliminate Joule

self-heating effects 2) Measure twice and average the two

resistances

VMU 2 (SMU 4)

SMU 2

VMU 1 (SMU 3)

SMU 1

R Force Current Twice (both ways), i.e. +/- IF

VM1 V

0 V

VMU 1 (SMU 3)

VM1

VEMF1

VOFF1 + - + -

VMU 2 (SMU 4)

VM2

VEMF2

VOFF2 + - + -

B2900A SMU SERIES FEATURES & BENEFITS

B2900A Series of Precision Source/Measure Units

B2900A Key Features: 1. Range of up to ±210 V and ±3 A (DC) / ±10.5 A (pulsed)

provides wider coverage for testing a variety of devices

2. Measurement resolution of 10 fA and 100 nV offers better source and measurement performance

3. GUI for quick bench-top testing, debug and characterization

Interactive Device Evaluation Can be Performed Entirely from the Front Panel (4 Viewing Modes):

Dual Channel View (2-ch Units Only) Single Channel View

Graph View (I-V, I-t, and V-t plots) Roll View (similar to strip chart)

Setting Up a Sweep Measurement from the Front Panel is Easy!

Speed

Show Sweep

Show Pulse

Show Trigger

More…

Assist Keys Sweep Settings Sweep Type

B2900A Series Sourcing Capabilities

Constant Linear Sweep Single Double

Log Sweep Single Double

List Sweep

DC

Pulse

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

V or I

t

Source Function

V or I

t

V or I

t

Arbitrary Waveform Generation The List Sweep function allows you to create arbitrary waveforms with up to 2500 steps. The timing resolution varies by B2900A model (20µs for B2901/02A, 10µs for B2911/12A ).

An icon appears in the GUI to indicate the type of sweep function selected.

B2900A Series Measurement Capabilities

High Speed Digitizing Capability In addition to its intrinsic measurement functions, the B2900A Series has an advanced trigger design that enables high speed digitizing measurements (20µs for B2901/02A, 10µs for B2911/12A).

V or I

t 20µs for B2901/02A 10µs for B2911/12A

:Measurement

The B2900A Series has four measurement functions that can be selected for either channel using its front-panel GUI or SCPI commands.

Voltage Measurement Current Measurement Resistance Measurement Power Measurement

Note: The Power Measurement function cannot be specified when using remote control

B2900A Maximum Voltage and Current Output

Maximum Voltage Maximum Current

DC or Pulsed

210 V 0.105 A

21 V 1.515 A2

6 V 3.03 A2

Pulsed only1

200 V 1.515 A

6 V 10.5 A

1. Maximum duty cycle is 2.5% 2. On 2-channel units some additional restrictions apply on the combined current

output of both channels (please refer to data sheet)

The B2900A Allows You to View Graphical Measurement Results

Roll View Provides a Convenient Way to View Slow-Moving Voltages/Currents Over Time

Can capture low-frequency “glitches”.

Agilent Has Free “Quick I/V” Software for Customers Wanting PC-Based Instrument Control

B2900A Series Model Comparison

Model # of SMUCh’s

Set/Measure Range Setting Resolution Measurement

Resolution Digitizing

Rate (sample/s)

Other Features Approximate

Pricing (US) Digits

Min. Resolution Digits

Min. Resolution

V I I V I V

B2901A 1

±210V

±3A/ch (DC) ±10.5A/ch (Pulsed)

5½ 1pA 1μV 6½ 100fA 100nV 50 k/sec (20µs/pt)

• XY View • AWG/List Sweep

$5.8K

B2902A 2 $8.5K

B2911A 1 6½ 10fA 100nV 6½ 10fA 100nV 100 k/sec

(10µs/pt)

• XY View • AWG/List Sweep • Roll View

$8.3K

B2912A 2 $12.8K

FINAL REMARKS

Additional Agilent SMU Instrument Products

Year 80 85 90 95 00 05 10

HP 4142B

4155/56 series 4145 series

E5260A/70B series

Modular SMU

Parameter Analyzer

Bench-top SMUs B2900A series

B1500A/B1505A

Agilent has more then 30 years of instrument SMU experience.

Device Analyzer

USB SMU

U2722A/23A N678xA SMU

Power Analyzer

http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng&nid=-33786.774878&imageindex=1

http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng&nid=-33786.937221&imageindex=1

http://www.home.agilent.com/agilent/product.jspx?nid=-35714.937223.00&cc=US&lc=eng

Comparison of the B1500A & B1505A

Software EasyEXPERT

– Tracer Test – Classic Test – Application Test – Quick Test

Modules MPSMU HRSMU ASU HV-SPGU WGFMU

Modules HCSMU HVSMU

Accessories SCUU GSWU

Accessories HV Bias-T Module Selector Unit

Max I/V - 1 A, 200 V Meas. Res. - 0.1 fA, 0.5 µV

Modules HPSMU MFCMU

B1500A B1505A

Max I/V - 40 A, 3000 V Meas. Res. - 10 fA, 2 µV

Parametric Measurement Handbook Available

You can download the PDF file (Rev 3) from the web: http://www.agilent.com/find/parametrichandbook You can also request it after completing the evaluation form.

>200 pages of invaluable information on parametric test

http://www.agilent.com/find/parametrichandbook

Question & Answer Session

the fundamentals of iv (current/voltage) measurement

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